Complex_I

From cppreference.com
Defined in header <complex.h>
#define Complex_I /* unspecified */
(since C99)

The Complex_I macro expands to a value of type const float _Complex with the value of the imaginary unit.

[edit] See also

the imaginary unit constant i
(macro constant)
(C99)
the complex or imaginary unit constant i
(macro constant)